1. Field of the Invention
The present invention relates to apparatus and a method for evaluating the performance of a heat exchanger. More particularly, the present invention concerns evaluating the heat transfer performance of a tube in tube type heat exchanger.
2. Description of the Prior Art
In many vapor compression refrigeration systems it has been found advantageous to exchange heat energy between refrigerant and water. One efficient type of heat exchanger for accomplishing this heat exchange is a tube in tube coaxial heat exchanger wherein an inner tube is mounted within an outer tube. When used in a refrigeration circuit the heat exchanger is connected to allow water to flow through the inner tube and refrigerant to flow through the outer tube such that heat energy may be transferred between the two.
An application for such a device may be found in the use of a water source heat pump wherein the water acting as a heat source in the heating mode is supplied through the outer tube of the heat exchanger or acting as the cooling sink in the cooling mode is supplied through the same portion of the heat exchanger. Another application could be the heating or chilling of water for either a hydronic cooling system or for a hot water heating system. The utilization of this type of heat exchanger as a combination refrigerant desuperheater and hot water preheater is also known in the industry. Such an application allows superheat energy contained in the refrigerant to be utilized for preheating hot water for a domestic hot water supply when an air conditioner is operating in the cooling mode.
It is particularly efficient to have tube in tube heat exchangers mounted within foamed insulation such that heat transfer between the outer tube and the ambient is prevented. A problem identified with such a heat exchanger is that it is difficult to evaluate the performance of the heat exchanger after foam insulation is supplied to surround the heat exchanger. In order to evaluate the heat transfer between the inner tube and the outer tube a specific scheme as set forth herein was developed.
The invention as set forth herein concerns supplying water at a predetermined temperature and flow rate to the inner tube of the heat exchanger. The water is then circulated through the inner tube of the heat exchanger and discharged into a separate heating unit at the other end of the heat exchanger. This separate heating unit contains heaters which act to supply thermal energy to the water at a constant rate. The now heated water is supplied to the outer tube of the heat exchanger and is circulated through the outer tube of the heat exchanger. Since the entering water temperature to the inner tube is controlled and the heat energy supplied between the inner tube and the outer tube is known, the overall amount of heat energy transferred between the inner and outer tubes may be calculated as a function of known constants and the temperature of the water being discharged from the inner tube. Hence, by the measurement of a single temperature it is possible to determine whether or not the heat transfer performance of the heat exchanger is suitable.